Various methods for controlling the output of steam turbines have been utilized in the past where the steam turbine forms part of a power installation including a steam generator for generating steam pressure and a superheater positioned between the steam generator and the turbine. The turbine itself is usually furnished with one or more control valves for varying the amount of steam supplied to the turbine to control its output.
Among the methods of controlling turbine output in subcritical steam pressure installations have been:
(1) constant or fixed throttle pressure-sequential valve operation in which the active nozzle area in the first stage of the turbine is varied by sequential operation of control valves to control flow of steam to the turbine;
(2) throttling control-single point valve operation in which all of the control valves are operated simultaneously and turbine output is varied by controlling the amount of throttling across the valves in order to control the flow of steam to the turbine;
(3) sliding pressure operation in which all the control valves are held in a fixed position and the throttle and nozzle inlet pressure are varied by varying the steam generator outlet pressure; and (4) hybrid operation which combines methods (1) and (3) or (2) and (3) at different ranges of turbine output.
Each method has its advantages and disadvantages when applied to commercial installations and their uses are dependent in part on steam generator and steam turbine design.
While the above methods of operation are applicable for use in subcritical installations, they may not entirely be used in supercritical installations utilizing steam pressures in excess of the critical pressure of steam, which is 3208.2 p.s.i. This is due in part to steam generator design considerations. In subcritical installations, sliding pressure operation is achieved by varying the firing rate of the steam generator which in turn varies the pressure of the steam delivered to the turbine. This method is not feasible for use in all supercritical operations because the pressure in the supercritical steam generators commonly used in the United States is never allowed to fall below the critical pressure whenever the steam generator is being fired.
It is therefore an object of my invention to provide for a method of varying turbine output in a supercritical steam generator-turbine installation which includes a sliding pressure control step over a portion of the range of turbine output whereby (1) temperature change and consequential thermal stresses imparted to the high pressure turbine may be minimized and whereby (2) net heat rate may be improved in lower load ranges of operation. The latter advantage can result in substantial fuel savings in the operation of a steam power plant.